1. Field of the Invention
The present invention relates to replication of original records and, more particularly, to a method of converting a video disc master which contains information in the form of microscopic "holes" in a metal surface into a master suitable for use in producing replicas.
2. Description of the Prior Art
Over the years, there has been a continuing attempt to achieve a low cost, mass produced disc which contains video information that can be retrieved with an inexpensive home instrument for playback through a conventional television set. Early attempts at providing video information have generally involved the use of video tape recorders of various sorts as well as photographic techniques. Still other approaches have attempted to utilize thermoplastic recording or the surface alteration of a thin metallic film.
In the copending application of John S. Winslow, Ser. No. 333,560, filed Feb. 20, 1973, and assigned to the assignee of the present invention, a method and apparatus was shown for producing a video disc master, which utilized a high power laser in conjunction with a glass disc having a thin film of a relatively low melting point material, such as bismuth, coated thereon. The laser beam was intensity modulated with the video information, and at the point of impingement upon the bismuth film surface, the laser beam, at relatively greater intensities, contained sufficient energy to melt the bismuth film.
The preferred physical properties of a low melting point material, such as bismuth, are that the surface tension of the melted material should cause the molten material to immediately coalesce into small, submicroscopic nodules, thereby leaving an area substantially free of the opaque metal coating. According to the Winslow application, typical "holes" representing video information were on the order of 1 micron.
The single master thus produced cannot, in and of itself, under prior art methods and techniques, be easily utilized to produce hundreds of thousands of replicas rapidly and at low cost. Accordingly, it has been deemed desirable to somehow modify the recorded master so that replication can be easily accomplished.
In the prior art, it has been known to create "masks" with a predetermined pattern which, in turn, could be used in connection with photoetching techniques to create a plurality of duplicate surfaces, each with a similar predetermined pattern in the surface. For example, such a mask could be utilized to selectively expose discs having a very thin metal surface coating to a laser beam, and a plurality of discs having a similar "hole" pattern in the surface could be produced.
Alternatively, a photoengraving process may be employed utilizing a master mask which, through chemical etching techniques, can result in a patterned disc.
Such techniques would not be directly applicable to the needs of the video disc system as presently envisaged because of the costs involved and the time required to create the duplicates or replicas. Since the pattern dimensions closely approximate the wavelength of visible radiation, normal, high speed photographic duplication techniques would be seriously affected by diffraction effects.